Skipping voting deadline in round editing/creation gives incomprehensible error

[lgelauff]

Reproduce: create a new round, or edit an existing round. Make sure all fields are valid, but leave the voting deadline empty.

This gives the following error:

Internal server error: <ExceptionInfo [ValueError: invalid literal for int() with base 10: ''] (41 frames, last=Callpoint('<listcomp>', 119, 'boltons.timeutils', '/data/project/montage-beta/www/python/venv/lib/python3.9/site-packages/boltons/timeutils.py', 12, ' dt_args = [int(p) for p in _NONDIGIT_RE.split(iso_str)]'))>

I would expect an error like "Choose a valid voting deadline".